1. Field of the Invention
The present invention relates to a method of integrated circuit assembly before encapsulation. More specifically, the present invention relates to soldering steps of such a method.
2. Discussion of the Related Art
In a conventional integrated circuit manufacturing method, each chip, resulting from the dicing of a semiconductive wafer on which are formed several chips, must be individually assembled on a heat sink. Each chip may be assembled on a connection grid, itself assembled on a heat sink, the sink and the connection grid being then separated by an isolator, for example, made of ceramic.
Each of the different elements (sink, ceramic, connection grid, chip) must be attached to at least another one of these elements. For this purpose, solderings have to be made between a first and a second element.
FIGS. 1A and 1B illustrate a conventional soldering method. An upper element 11 (for example, a chip) is desired to be attached to a second lower element 12 (for example, a grid) by means of a solder paste 13. Initially, as illustrated in FIG. 1A, first element 11 is laid on solder paste 13. The distance separating the first and second elements 11, 12 thus corresponds to thickness T1 of deposited paste 13, for example, about 40 .mu.m. Such a paste is typically formed of balls 14 of fusible metals such as lead, tin, and silver in various combinations.
In the automatic deposition of the different assembly elements (chip, ceramic, grid), a mechanical pressure is exerted vertically from top to bottom, this pressure being generally high due to the absence of any clearance in the grasping system: the absence of any clearance is compulsory for a good positioning accuracy upon deposition.
This pressure "crushes" the joint before the fusion, the two elements to be soldered being then separated from each other by a minimum distance equal to the maximum diameter of the paste particles, that is, about 40 .mu.m. Another disadvantage of this pressure upon deposition is to expel the paste towards the outside of the surface to be soldered.
Then, paste 13 is brought to the fusion point while exerting, on first element 11, a light mechanical pressure homogeneously distributed on its surface.
FIG. 1B illustrates the assembly obtained by such a method. Conventionally, first and second elements 11, 12 are now only separated by a soldering joint (hard soldering) 16 of a thickness T2, for example, on the order of 15 .mu.m, substantially one third of initial thickness T1 of paste 13.
A disadvantage of this method is linked to the very small final thickness T2, especially as compared to the contact surfaces involved, which are relatively large. Indeed, in the operation at high temperatures and during temperature variations, the heat coefficient difference between the two elements of different nature attached by soldering, combined with the small thickness of the soldering, can cause disconnection of the two elements. Such disconnection results in malfunctions of the circuit in which it occurs.
Thickness T2 of the obtained soldering depends on the thickness T1 of initially deposited solder paste. It has already been provided to increase thickness T1, by increasing the amount of deposited solder paste. But the high pressure during the deposition of the elements expels the paste outside the surface to be soldered, which results in new disadvantages, especially a problem of alignment of the elements, which tend to float.
It should be noted that these problems also arise when a soldering joint is created between a ceramic pad and a connection grid and between this pad and a sink. Indeed, if elements like the connection grid and the sink appear, upon assembly, in the form of strips of linked elements, the chips and ceramic pads are however individualized before this assembly and are thus not held laterally.